Effects of ethylenic bond position upon acyltransferase activity with isomeric cis-octadecenoyl coenzyme A thiol esters.

The specificity of the acyl-CoA :phospholipid acyltransferases has been studied using the r6 positional isomers of cis-octadecenoic acid. The results showed that the acyl-transferases acting at both the Iand z-positions of acyl-glycero-3-phosphorylcholine (acyl-GPC) discriminated between the acyl-CoA isomers in quite different ways. I. Acyl-CoA:r-acyl-GPC acyltransferase activity showed a distinct preference for the 9-, and xz-isomers. Of these three, the 9-~ctadecenoate (oleate) was the preferred substrate having a rate of 98 nmolesjmin per mg. 2. Acyl-COAX-acyl-GPC acyltransferase reacted more rapidly with the B-, IO-, IZ-, 13and Is-isomers, and of these the xz-octadecenoate had the fastest rate (IZI nmolesjmin per mg). 3. As the enzymes were allowed to age at 4O, the activity was lost at slightly different rates for each isomer. The enzyme(s) esterifying the x-position seemed to loose activity fairly uniformly with all isomers so that a similar pattern of reactivities was observed over a period of several days. The enzyme(s) esterifying the z-position, however, differed in that after 2 days, the rate for the 9-isomer had dropped below that for the rz-isomer. This result suggests that different enzymes may exist for different acyl-CoA isomers. 4. High concentrations of sucrose (0.8 M) tended to stabilize the activities with the 9and rz-isomers, but did not change the fact that the activity for the 9-isomer was lost more rapidly. Albumin, contrary to our expectations, increased the rate of loss of activity. 5, The enzyme activities were purified IOto rj-fold above that of the crude tissue homogenate by treating the microsomal particles with sodium deoxycholate and albumin.